This invention pertains to the application of lamella (or tube) settlers to crystallizer design in general; in particular, application of settlers to compact crystallizers such as used in advanced type batteries and power cells for electric vehicle or stationary applications, and more particularly to a new approach for solving the hydrargillite removal problem from aluminum-air batteries.
The beneficial effect on settling of suspensions by having inclined rather than vertical tubes is well known. In current practice, the inclined plate settlers, commonly known as lamella settlers, consist of parallel plates, usually arranged in a stacked array, which form channels into which a slurry is fed. Under the action of gravity, the solids deposit on the upper surface of the inclined plate at the bottom of each channel and flow to the dense slurry collector at the bottom of the settler. The clear liquid rides under the lower surface of the plate at the top of each channel and is collected at the top of the lamella settler.
Lamella settlers are very effective, low energy intensity sedimentation devices which are being extensively used in water treatment plants and some applications in the mining and minerals industry. Lamella settlers can be used in place of other commonly used separation devices, such as hydrocyclones, elutriators, thickeners, etc. While lamella settlers do not provide as sharp a separation of fine and coarse particles as hydrocyclones, when used as clarifiers they have a number of advantages over hydrocyclones, such as low energy consumption and low shear.
Lamella settlers are exemplified by U.S. Pat. No. 4,151,084 issued in Apr. 1979 to R. F. Probstein et al, as well as by numerous publications, such as "Lamella and Tube Settlers. 1. Model and Operation", W. Leung et al, Ind. Eng. Chem. Process Dev., 22, 58-67 (1983), and "The Sedimentation of Polydisperse Suspensions In Vessels Having Inclined Walls", R. H. Davis et al, Int. J. Multiphase Flow, 8(6), 571-585 (1982).
Crystallization is a well known technology and various types of crystallizers are known in the art. However, one of the aspects of crystallization, particularly when applied to small, compact systems, such as required for successful application of advanced batteries/power cells to electric vehicles, is the control of the particle size distribution of the crystals. Very fine particles are continuously being formed by breakage of larger crystals and by secondary nucleation; both of these are promoted by high shear conditions.
The control of fines is a general problem in industrial applications of crystallization. In industrial applications the control of fines is usually performed on a side stream containing only fine particles suspended in the mother liquor; the suspension is heated above the temperature required for complete solids dissolution, cooled and returned to the crystallizer. However, this means of fines control/removal is of limited applicability to compact, mobile systems; in the case of the aluminum-air power cell the solutions are highly supersaturated and heating to 150.degree.-230.degree. C. would be required for fines removal. The equipment required to obtain such heating and holding the suspension at elevated temperature and pressure would add a prohibitive penalty to the weight, volume and cost of such a power cell system.
Another way to remove the excess fine crystals is by agglomeration, a process where small crystals coalesce and are bonded together to form a larger agglomerate. Experimental work on agglomeration in suspension has pointed out the necessity for low shear in the agglomerating process; the initial agglomerates are very fragile and break-up very easily.
A problem arising in the design of electrochemical reactors involving suspended solids (or two liquid phases of different densities) is that there is a need to:
1. Maintain a low solids (or second phase) concentration in the cells. PA1 2. Obtain efficient contacting between the two phases (solid-liquid or liquid-liquid) in a separate tank. PA1 3. Minimize energy consumption in all auxiliary devices.
Thus, separation devices of some sort are required. An additional problem in the aluminum-air and lithium-air battery is the removal of the reaction product from the system.
The prior development efforts of the aluminum-air power cell, for example, utilized hydrocyclones for the removal of most of the solids from the electrolyte flow returning to the cell stack and for coarse product removal. Such an arrangement utilizing cyclone separators is described and illustrated in document UCID-20356 entitled "Aluminum-Air Power Cell Research and Development Annual Report Summary, CY 1984", A. Maimoni, Feb. 27, 1985. However, as recognized in the art, cyclone type systems involve high shear effects and involve relatively high energy consumption, with the high shear adversely affecting crystal growth and agglomeration.
Thus, there is a need for an efficient, low shear, low energy consumption particle separation process. In addition, in aluminum-air batteries/power cells there is a need for an efficient and effective means for coarse product separation and removal, and to provide for crystal growth and agglomeration.
Therefore, it is an object of this invention to provide an apparatus for particle separation/crystallization which utilizes low shear and low energy consumption.
A further object of the invention is to provide a particle crystallization means which incorporates a lamella settler.
A further object of the invention is to provide a new approach for solving the hydrargillite removal problem from aluminum-air batteries.
A still further object of the invention is to provide an apparatus which combines lamella settlers and crystallizers for use in compact batteries and power cells for improved removal of reaction products.
Another object of the invention is to provide a lamella settler crystallizer, particularly adapted for use in electrochemical systems requiring separation of phases of different densities.
Another object of the invention is to provide an improved crystallizer for separation of solid reaction products from liquids which incorporates a lamella settler.
Another object of the invention is to provide a lamella settler crystallizer which operates in a low shear environment, minimizes breaking of crystals by abrasion, and is essential to the formation of agglomerates, while operating at low pumping costs.
Still another object of the invention is to provide a lamella settler crystallizer which is particularly adapted for aluminum-air batteries/power cells, and includes a mechanism for coarse product separation and removal while maintaining the coarse fraction of the crystal population within the crystallizer, and providing a dilute suspension of fine particles for return to the cell while retaining the particles in the agglomerating range in a low shear environment.
Other objects and advantages of the invention will become apparent to those skilled in the art from the following description and accompanying drawings.